In the making of micro-circuits the general process followed is that of generating an oxide film on the semiconductor substrate; coating the oxide film with a photoresist and then exposing the photoresist, through a mask, to radiation. The mask has a pattern of opaque or transparent portions thereon. After exposure, the photoresist is developed creating a pattern of exposed or unexposed portions corresponding to the pattern of transparent or opaque portions of the mask.
The photoresist that remains after developing forms a protective cover for the oxide layer. The exposed portion of the oxide layer is then etched to expose portions of the substrate layer below it. Impurities can then be doped into the substrate to give it electrical properties characteristic of the particular dopant/substrate combination created. Once done, the process may be repeated a number of times with additional oxide films formed, more resist deposited, the resist exposed, developed, and the oxide film etched with further steps of doping or depositing carried out.
Separate masks may be used for each of the successive exposure steps. If circuit elements in successive layers are to be properly aligned or registered with each other a high degree of alignment must be maintained between layers.
Almost all of the current automatic alignment systems require that the mask and wafer be essentially fixed with respect to the alignment system during the alignment process. This is no problem in a step-and-repeat type system, but is far from ideal in a step-and-scan projection printer as used in the present invention since misregistration can occur when the mask and wafer move with respect to each other and with respect to the projection system.
A promising approach to automatic wafer-to-reticle alignment is offered by the dark-field alignment concept. It was recognized early in the development of automatic wafer-to-reticle alignment systems that the only reliable system capable of working with complex topographies, covered with photoresist, had to be based on dark field imaging. This concept, generally, calls for alignment marks, or wafer targets, on the wafer to be illuminated by an alignment illumination source. Light from the alignment illumination source typically floods the wafer surface and is backscattered by edges of the wafer target and reflected by the wafer target itself. The intensity and position of backscattered radiation is detected and compared with the position of alignment features on the reticle to determine the degree of alignment or misalignment between the mask and reticle.
While a number of different types of dark-field wafer-to-reticle alignment systems have been provided in the prior art, and operate with moderate success, the reverse dark-field system is an improvement over such prior art dark-field systems. The improvement of the reverse dark-field alignment system over the prior art dark-field system is expected because the method has inherently lower background light levels, is less dependent on the quality of the light collection optics, and because symmetrical illumination can easily be achieved. A reverse dark-field system is also less susceptible to process induced variations of target profiles.
Early applications of the reverse dark-field concept to wafer-to-reticle alignment were found in step-and-repeat photolithography systems. See, e.g., R. S. Hershel, SPIE vol. 221, pp. 34-43 (1979). Such applications required relative motion between the mask and wafer in order to obtain an alignment measurement, which necessarily precluded alignment during exposure. Subsequent applications of the dark-field alignment concept were found in scanning exposure systems. See, e.g., U.S. Pat. No. 4,301,363; A. Suzuki, SPIE vol. 275, pp. 35-42 (1981). These systems scanned a laser beam across stationary alignment targets and collected scattered light for use in determining the alignment error. In such applications the mask and reticle are stationary during the alignment measurement process while the laser beam is actively scanned by a rotating mirror. This system is not configured for simultaneous exposure and alignment nor does it make use of mask and wafer scanning motions.
My contribution to the art allows the foregoing advantages to be incorporated into scanning photolithographic equipment.